1. Field of the Invention
The present invention concerns a method for regulating a transmission unit for motor vehicles, as well as a transmission unit applying such a method.
In particular, the invention concerns a method for regulating clutch pressures of an engine drive coupling in transmission units which are provided with a belt variator.
2. Description of the Related Art
As is known, such a belt variator comprises: a primary pulley placed on an input shaft; a secondary pulley placed on a driven or output shaft; and an endless transmission element provided between both pulleys. Each pulley is formed of two conical pulley halves which may be moved in relation to one another such that the transmission ratio may be altered by pulling the pulley halves more or less apart. The mutual position of the pulley halves is set as a function of various data by hydraulic pressure cylinders which are controlled by regulating means.
The belt variator is coupled to the motor of a motor vehicle via a drive coupling including a reversing unit. The belt variator is connected to the wheels of the vehicle by the driven shaft, such that, by the drive coupling, which includes a forward clutch coupling and a reverse clutch coupling, either a driving connection between the motor and the belt variator may be broken, a connection may be made in the forward driving direction, or a connection may be made in the reverse driving direction.
The forward coupling and the reverse coupling comprise flexible disc clutch couplings which are driven or actuated by fluid pressure. The control of the clutches may be carried out according to known embodiments in a hydraulic or electronic manner.
With the known hydraulically controlled transmission units, the drive coupling is controlled by a clutch pressure which is mainly determined by the engine speed. This may be done via an engine Pitot pressure which, as is known, depends quadratically on the engine speed.
With known entirely electronically controlled transmission units, the clutch pressure is set as a function of various parameters which are processed in an electronic manner. To this end, rather expensive electronic hardware is required. Moreover, a hydraulic backup system must be provided which assumes a minimal number of functions in case the electronics fail. Thus, the hydraulic control unit is still very complex even though the control is carried out electronically.